We report a case of Epstein-Barr virus infection with the subsequent development of haemophagocytic lymphohistiocytosis in a teenage Crohn's disease patient treated with azathioprine. We found that the early introduction of the anti-B cell monoclonal antibody rituximab precipitated a rapid fall in circulating B-cells and EBV viral load, resulting in a prompt and sustained recovery from what is a potentially fatal complication of azathioprine therapy in Crohn's disease patients.